2. Go to Forum list, and make note of any category URL.
3. Disable the "Create Pages for Categories" setting.
4. Visit the URL you noted down in #2.
Fatal error: Cannot use object of type XenForo_Link as array in /library/XenForo/Controller.php on line 513
Traced it down to XenForo_Route_Prefix_Categories::buildLink() which returns a XenForo_Link object if "categoryOwnPage" option is false. It should return a plain string.